1. Field of the Invention
The present invention relates to a semiconductor device comprising a transistor, such as a power transistor or an insulated gate bipolar transistor (IGBT), for use as a high speed semiconductor power switch.
2. Discussion of the Related Art
Typically, when a transistor, such as a power transistor or an IGBT, is used as a high speed power switch, a diode, such as a free wheeling diode (FWD), is connected across the transistor's emitter and collector to provide a current path from the transistor's emitter to its collector. For example, as shown in FIG. 3, there is provided a transistor 1 having a high speed diode 2 reverse-bias connected between a collector 12 and emitter 11 of the transistor 1. As shown in FIG. 2, such a circuit arrangement has application in an inductive load circuit, wherein a plurality of power rectifiers 22 and transistor inverters 23 control the speed of a motor 21.
In the circuit arrangement shown in FIG. 3, it is often desirable that the high speed diode 2 not only provide a current path from the transistor's emitter to its collector, but also function as a constant voltage diode to protect the transistor 1 against voltage surges at the collector 12. Thus, it is often desirable that the high speed diode 2 have the characteristics of, for example, a Zener or avalanche diode, as exemplified by Zener diode 3 shown in the equivalent circuit of FIG. 4. It is known in the art that a reach-through Zener diode having a cross sectional structure shown in FIG. 5 may be used as the Zener diode 3 shown in the circuit of FIG. 4.
As shown in FIG. 5, the reach-through Zener diode comprises a cathode electrode 36, an N.sup.+ -type layer 32 formed on the cathode electrode 36, an N.sup.- -type layer 31 formed on the N.sup.+ -type layer 32, a P.sup.+ -type layer 33 diffused in the N.sup.- -type layer 31, an oxide film 34 formed on the P.sup.+ -type layer 33, and an anode electrode 35 formed on the P.sup.+ -type layer 33 through an opening in the oxide film 34. The Zener voltage of the reach-through Zener diode is determined by the specific resistance which is largely a function of the width W1 of the N.sup.- -type layer 31. Because t.sub.rr of such a Zener diode is long, however, the diode will not have the desired characteristics of the high speed diode 2 shown in FIG. 3. Furthermore, if a lifetime Killer is used to control t.sub.rr of the Zener diode to shorten t.sub.rr, the Zener voltage will not be stable. Where, t.sub.rr indicates a time in which a reverse current flowing a diode when a voltage applied to the diode is changed from a fixed forward voltage to a fixed reverse voltage, recovers to 10% of the maximum value.